The present invention relates to a process for removing sulfur compounds from hydrocarbon feedstreams, particularly those boiling in the naphtha range by contacting the feedstream with an adsorbent comprised of cobalt and one or more Group VI metals selected from molybdenum and tungsten on a refractory support. This invention also relates to a process wherein a naphtha feedstream is first subjected to selective hydrodesulfurization to remove sulfur but not appreciably saturate olefins. A product stream is produced containing mercaptans that are removed by use of the cobalt-containing adsorbents of the present invention.
The presence of sulfur compounds in petroleum feedstreams is highly undesirable since they result in corrosion and environmental problems. These compounds are also responsible for reducing the performance of engines using such fuels. It has not been considered prudent in the past to transport refined hydrocarbon fluids in a pipeline previously used for the transportation of sour hydrocarbon fluids, such as petroleum crudes. The major difficulty is that refined hydrocarbon fluids, such as gasoline and diesel fuel, pick up contaminants such as elemental sulfur. About 10 to 80 mg/L of elemental sulfur is picked up by gasoline and about 1 to 20 mg/L elemental sulfur is picked up by diesel fuel when pipelined. Elemental sulfur has a particularly corrosive effect on equipment, such as brass valves, gauges, silver bearing cages in two-cycle engines and in-tank fuel pump copper commutators.
The maximum sulfur level allowable in gasoline in the U.S. is 350 wppm. In 2004, the sulfur level in motor gasoline will be legislated to less than 30 wppm. Auto emissions into the environment is one of the highest sources of atmospheric contaminants.
Refiners have a number of options to produce lower sulfur gasoline. For example, they can refine lower sulfur crudes, or they can hydrotreat refinery streams to remove contaminants via processes such as adsorption and absorption.
Hydrodesulfurization is the conventional method for removal of sulfur compounds from hydrocarbon streams. In typical hydrodesulfurization processes, a portion of the sulfur components is removed from a hydrocarbon feed stream by reaction of the sulfur components with hydrogen gas in the presence of a suitable catalyst to form hydrogen sulfide. The reactor product is cooled and separated into a gas and liquid phase, and the off-gas containing hydrogen sulfide is discharged to the Claus plant for further processing. Hydrodesulfurizing processes that treat FCC gasoline, the major sulfur source in U.S. refinery gasoline, are characterized by both an undesirable high rate of hydrogen consumption (due to olefin saturation) and a significant octane degradation. Also, these processes require severe conditions, such as high temperatures up to about 425xc2x0 C. as well as pressures up to about 3000 psig.
Selective and severe hydrodesulfurization processes have also been developed to avoid extensive olefin saturation and octane loss. Such processes are disclosed, for example, in U.S. Pat. Nos. 4,049,452; 4,149,965; 5,525,211; 5,243,975 and 5,866,749. However, in these and other such processes, H2S reacts with the retained olefins in the hydrodesulfurizaton reactor and forms mercaptans. Depending on the amount of sulfur and olefins in the naphtha feedstream, the concentration of these reversion reaction product mercaptans typically exceeds fuel specifications for mercaptan sulfur and, in some cases, total sulfur as well. Therefore, removal of these mercaptans is essential to meeting the future fuel specifications with regard to sulfur level, particularly with respect to mogas pool stocks.
Gonzales et al. (xe2x80x9cCan You Make Low-Sulfur Fuel and Remain Competitive,xe2x80x9d Hart""s Fuel Technology and Management, Nov/Dec 1996) indicates that cat feed desulfurization can reduce sulfur levels in cracked naphtha to 500 wppm. However, this is an expensive option, especially if a refiner cannot take advantage of the higher gasoline conversions as a result of cat feed desulfurization. Sulfur levels lower than 200 wppm are achievable via hydrodesulfurizaton of light cracked-naphtha. However, this is incrementally even more expensive than cat feed desulfurization because of the high hydrogen consumption and loss of octane due to hydrogenating the olefins. Thus, the hydrotreated cracked-naphtha needs to undergo an isomerization step to recover some of the octane.
Caustic extraction processes, such as the Merox process, is capable of extracting sulfur from hydrocarbon feedstreams, which sulfur is in the form of mercaptan compounds. The Merox process was announced to the industry in 1959. The Oil and Gas J. 57(44), 73-8 (1959), contains a discussion of the Merox process and also of some prior art processes. The Merox process uses a catalyst that is soluble in caustic, or alternatively is held on a support, to oxidize mercaptans to disulfides in the presence of oxygen and caustic. Mercaptans are corrosive compounds that must be extracted or converted to meet an industry standard copper strip test. Sodium mercaptans are formed which are soluble in caustic solution. The caustic solution containing the mercapatan compounds is warmed and then oxidized with air in the presence of a catalyst in a mixer column that converts the mercaptan compounds to the corresponding disulfides. The disulfides, which are not soluble in the caustic solution, can be separated and recycled for mercaptan extraction. The treated hydrocarbon stream is usually sent to a water wash in order to reduce the sodium content.
Such caustic extraction processes, however, are capable of extracting sulfur only in the form of light mercaptan compounds (for example, C1 to C4 mercaptans) that typically accounts for less than about 10% of the sulfur present in na FCC gasoline. Problems associated with caustic extraction include: generation of hazardous liquid waste streams, such as spent caustic (which is classified as hazardous waste); smelly gas streams which arise from the fouled air effluent resulting from the oxidation step; and the disposal of the disulfide stream. Further, Merox processing problems include difficulties associated with handling a sodium and water contaminated product. Caustic extraction is able to remove only lighter boiling mercaptans while other sulfur components, such as sulfides and thiophenes, remain in the treated product streams. Also, oxygen compounds (e.g., phenols, carboxylic acids, peroxides) and nitrogen compounds (e.g., anines or nitrites) also found in FCC gasoline are not appreciably affected by the Merox process.
Adsorption is often a cost-effective process to remove relatively low levels of contaminants. Salem, A. B. et al., (xe2x80x9cRemoval of Sulfur Compounds from Naphtha Solutions Using Solid Adsorbents,xe2x80x9d Chemical Engineering and Technology, Jun. 20, 1997) reports a 65% reduction in the sulfur level (500 to 175 wppm) for a 50/50 mixture of virgin and cracked naphthas using activated carbon at 80xc2x0 C. and a 30% reduction using Zeolite 13xc3x97 at 80xc2x0 C. Also, U.S. Pat. No. 5,807,475 teaches that Ni or Mo exchanged Zeolite X and Y can be used to remove sulfur compounds from hydrocarbon streams. Typical adsorption processes have an adsorption cycle whereby the contaminant is adsorbed from the feed followed by a desorption cycle whereby the contaminant is removed from the adsorbent.
In spite of limitations, the above mentioned processes, for the most part, provide satisfactory means for reducing the level of sulfur in refinery hydrocarbon feed streams to levels that were previously acceptable. These processes are not, however, suited for the economic reduction of heteroatom contaminants to the substantially lower levels that are now or will soon be required by governmental regulations. Thus, there is a need in the art for processes that can meet these ever stricter regulations.
In accordance with the present invention, there is provided a process for removing sulfur compounds from sulfur compound-containing hydrocarbon streams, which process comprises contacting a sulfur-containing hydrocarbon stream with an adsorbent comprised of Co and at least one Group VI metal selected from Mo and W on an inorganic support under conditions that include temperatures up to about 150xc2x0 C., in the substantial absence of added hydrogen.
Also in accordance with the present invention there is provided a process for removing sulfur from sulfur compound-containing naphtha streams, which process comprises:
(a) hydrodesulfurizing said naphtha stream, which contains olefins and sulfur in the form of organic sulfur compounds, to form a hydrodesulfurization effluent at an initial temperature, the hydrodesulfurization effluent comprising a hot mixture of sulfur reduced naphtha at an initial pressure, H2S and mercaptans, and then
(b) contacting said mixture with an adsorbent comprised of Co and at least one Group VI metal selected from Mo and W on an inorganic support under conditions that include temperatures up to about 150xc2x0 C., in the substantial absence of added hydrogen.
In a preferred embodiment of the present invention there is provided, between step (a) and step (b) a step wherein the system is rapidly depressurized for a depressurization time at least a portion of the hydrodesulfurization effluent to destroy at least a portion of the mercaptans to form more H2S and a depressurized naphtha further reduced in sulfur
In another preferred embodiment, the hydrocarbon stream is a naphtha boiling range petroleum stream.
In still another preferred embodiment, the inorganic support is selected from alumina, silica, and large pore zeolites.
In yet another preferred embodiment, the adsorbent contains from about 0.5 to about 20 wt. % Co and about 1 to about 40 wt. % of Mo and/or W.
In still another preferred embodiment, the adsorbent is preconditioned with H2.
In another preferred embodiment, the adsorbent is preconditioned with a mixture of H2S and H2.